Clarification of phosphoric acid

ABSTRACT

Phosphoric acid in a wet phosphoric acid process is clarified by introducing into the acid a small but effective amount of a clarification additive which is a sulfonated polystyrene or a water soluble salt thereof having a molecular weight from about 1 to about 40 million and sulfonated in the para-position in the benzene ring in an amount of about 1 to 3 moles per mole of benzene ring, the clarification additive being added to the acid in one or more zones of the process.

' [73] Assignee:

United States Patent Naschke et al.

[ Feb. 22, 1972 [54] CLARIFICATION OF PHOSPHORIC ACID [72] Inventors:John H. Naschke; Robert L. Taylor, both of Houston, Tex.

[52] U.S. Cl ..23/165, 210/54 [51] Int. Cl. ..C0lb 25/18, C01b 25/22[58] Field ofsearch ..23/165, 165 B,3l2D;210/54; 209/5 [56] ReferencesCited UNITED STATES PATENTS 2,955,919 10/1960 Wilson ..23/165 3,009,87311/1961 Kerr etal ..210/53 3,300,407 l/ 1967 Priesing et al. ..210/54FOREIGN PATENTS OR APPLICATlONS 967,607 8/1964 England ..210/54 PrimaryExaminerEarl C. Thomas Assistant Examiner-Gregory A. HellerAitorney-Thomas B. McCulloch, Melvin F. Fincke, John S. Schneider,Sylvester W. Brock, Jr., Kurt S. Myers and Timothy L. Burgess [57]ABSTRACT Phosphoric acid in a wet phosphoric acid process is clarifiedby introducing into the acid a small but effective amount of aclarification additive which is a sulfonated polystyrene or a watersoluble salt thereof having a molecular weight from about 1 to about 40million and sulfonated in the para-position in the benzene ring in anamount of about 1 to 3 moles per mole of benzene ring, the clarificationadditive being added to the acid in one or more zones of the process.

13 Claims, 2 Drawing Figures PAIENTEBFEBZZIQYZ 3.644.091

sum 2 OF 2 35 CLARIFICATION ADDITlVE FOAM INHIBITOR ll 1 ACID SLURRYCOOL DOWN TANK ROCK FEED CLARIFICATION I 37 ADDITIVE 33 RECYCLE ACID 34H so 2 4 FOAM 35 CLARIFICATION ADDITIVE 35a 350 CLARIFICATION 35 ifigrADDITIVE 1 FOAM mmsnon INVENTOR 5.

JOHN H. NASCHKE,

BY ROBERT L. TAYLOR,

gI: ORNEY.

CLARIFICATION OF PHOSPHORIC ACID BACKGROUND OF THE INVENTION 1. Field ofthe Invention The present invention is directed to clarification ofphosphoric acid. More particularly, the invention is concerned withclarification of phosphoric acid by flocculation or coagulation ofsolids. In its more specific aspects, the invention is concerned withclarification of phosphoric acid in a wet process in which phosphaterock is treated with sulfuric acid and the resulting acid is clarifiedby addition of a particular clarification additive.

2. Prior Art Polyelectrolytes or polymer coagulants known to the artsuch as polysaccharides, leguminous seed derivatives, polyacrylamides,polyacrylonitriles, polyacrylic acid and salts thereof, compoundscontaining polyamide and carboxylic groups, polyamines, polyethyleneoxide, algin and its salts, sodium carboxymethylcellulose, starchderivatives, sulfonated polyacrylamide and its sodium salts, mixturesthereof have been found to be noneffective in coagulating dispersedsolids in the wet phosphoric acid process, which process is well known.However, it has now been found that sulfonated polystyrene sulfonated inthe paraposition ofthe benzene ring in an amount of about I to about 3moles per mole of benzene ring and having a molecular weight within therange of about l to about 40 million effectively coagulates solids inthe wet phosphoric acid process and clarifies the phosphoric acid. Thewater-soluble salts such as the alkali metal salts of the sulfonatedpolystyrene also are effective.

SUMMARY OF THE INVENTION The present invention may be briefly describedand summarized as involving the introduction of a clarification addiliveinto the acid in one or more of the zones of a wet phosphoric acidprocess. Thus, the clarification additive which is a sulfonatedpolystyrene or a water soluble salt thereof having a molecular weightwithin the range of about l to about 40 million may be introduced intothe digestion zone,

the acid introduced into the evaporation zone or into the acid leavingthe evaporation zone and introduced into a centrifugation zone or intothe acid leaving the centrifugation zone or into the storage zone. Theclarification additive may be introduced directly into the zones or intothe phosphoric acid in or introduced into the zones. It may beintroduced into one or all of the zones or it may be introducedincrementally into one or more zones.

VARIABLES OF THE INVENTION The clarification additive employed in thepresent invention is sulfonated polystyrene which has a high molecularweight in the range between about 1 million and about 40 million,preferably above about 3 million and in the range up to about 12million. The water soluble salts of the sulfonated polystyrene havingmolecular weights in the ranges given may also be used and include thealkali metal salts such as sodium, potassium and lithium salts with thesodium salt being preferred over the other salts. The sulfonatedpolystyrene and its water soluble salts are sulfonated in theparaposition of the benzene ring with a degree of sulfonation of about 1mole per mole benzene ring. Sulfonated polystyrene of about 6 millionmolecular weight give desirable results.

The clarification additive may be added in an amount within the rangefrom about 10 to about 200 ppm. of the phosphoric acid to be clarified.This amount may be added incrementally to at least one zone or may beadded to several zones of the wet phosphoric acid process eitherdirectly or to the phosphoric acid introduced into the zones. Thus, theclarification additive may be added directly to the digestion zone or tothe acid introduced to or into the clarification zone, the evaporationzone, or the centrifugation zone, or after the cen trifugation zone, orto all zones as may be desired.

A foam inhibitor may be used in conjunction with the clarification agentand also introduced into one or more zones of the wet phosphoric acidprocess. A preferred foam inhibitor may be a homogeneous mixture ofpolyoxyethylene sorbitan ester of a fatty acid and a fatty acid. Thefatty acid may be tall oil fatty acid. However, other foam inhibitorsmay be used such as but not limited to sodium stearate and petroleumbased pure oil.

The conditions employed in the digestion zone of the system of thepresent invention include a temperature within the range of 70 to about85 C. with a preferred temperature of about 75 C. The temperature'in theflash cooler and filter zone may range from about 60 to 70 C. dependenton the temperature in the digestion zone. In the clarification zone thetemperature may range from about 60 to about 70 C. with a preferredtemperature of about 60 to about 63 C. In the multiple stageevaporators, the pressure may range from about 2 to 8 inches of mercuryand temperatures from to C. Temperatures in the centrifugation zone mayrange from about 80 to about 85 C.

The phosphoric acid may contain from about 0.5 percent to about 30percent by wt. of solids. Effective clarification has been obtained withphosphoric acid containing from about 2 to about 27 percent by wt. ofsolids.

BRIEF DESCRIPTION OF THE DRAWING The present invention will be furtherillustrated by the drawing in which:

FIG. 1 is a flow diagram of a preferred mode and embodiment; and

FIG. 2 is a view of FIG. I taken along the lines 2- 2.

DESCRIPTION OF THE PREFERRED MODE AND EMBODIMENTS WITH RESPECT TO THEDRAWING Referring now to the drawing and particularly to FIGS. 1 and 2,numeral 11 designates a digestion zone comprised of a plurality ofstages 12, I3, 14, 15 and 16 defined by baffles 17, 18, 19, 20 and'2l. Acircular well 22 is formed by cylindrical wall member 23. Each of thestages l2, 13, 14, 15 and 16 is provided with agitating means 24provided with stirring means 25 and 26 arranged on shafts 27 actuated byprime movers such as electrical motors 28. The circular well 22 is alsoprovided with an agitating means 29 arranged on a shaft 30 and driven byprime mover 31. Introduced in the stage 12 by conduit 32 is dried,ground phosphate rock from a source not shown and having a typicalcomposition such as set out in Table I.

TABLE I Analysis of Acidulation-Grade Phosphate Rock Samples Thisanalysis has been adjusted to 100 percent. It is understood that otherphosphate rock may be used.

The stages 12, 13, 14, and 16 formed by baffles 17, l8, 19, 20 and 21are formed such that the rock slurry may proceed from one stage to theother; thus, baffle 12 reaches to the bottom of digestion zone 11 whilethe baffle plate 21 provides a passageway such as 32 from stage 15 to16. Thus, baffle 18 would be like baffle 21, etc. Where a baffle reachesto the bottom of zone 11, the slurry overflows into the next stage.Leading into stage 12 is recycle acid by way of line 33 from a sourcewhich will be described further. Sulfuric acid having a strength in therange from 54 to 95% H 80, is introduced by line 34 and foam inhibitoris introduced into the several stages by lines 35 controlled by valves36. The clarification additive of the present invention is introducedinto the several stages by lines 35a controlled by valves 36a. Inasmuchas the clarification additive is effective in the digestion zone 11,means such as line 22a controlled by valve 22b is provided for removingany rock or solids which are flocculated in zone 11. The resulting acidslurry is withdrawn from stage 16 by line 37 and discharged thereby intoa cool down tank 38 where the temperature is suitably reduced to atemperature in the range given before. Recycle acid is withdrawn fromcool down tank 38 by line 33 as has been described. Acid is alsowithdrawn and introduced into line 33 by branch line 39 which connectsto cool down tank 38. The cooled acid discharges from cool down tank 38by line 40 and is introduced thereby into a 3 to 4 stage dump-typefilter zone 41 where phosphoric acid is separated from the gyp filtercake and the filter cake is withdrawn by line 42 for further treatmentand disposal.

Acids of various concentrations are withdrawn from the several stages offilter zone 41. Thus, the strongest acid is withdrawn by line 42 into ahold tank 44 and thence into a clarification zone 45 by line 46.Clarification additive is introduced into line 43 by line 43a controlledby valve 43b. The acid stream contains rock which is separated from theacid in clarification zone 45 and disposed of by withdrawing same byline 45A. The clarified acid is then introduced by line 47 into amultiple stage evaporator 48 operating at a suitable temperature andreduced pressure to remove water by line 49; foam inhibitor may be addedby line 50 controlled by valve 51. Phosphoric acid at a concentration ofabout 54 percent by weight is recovered by line 52.

Alternatively, hold tank 45 and zone 46 may be bypassed. This may bedone by closing valve 43c in line 43 and allowing the acid in line 43 toflow by line 8 controlled by valve 9 directly to line 47. Under thesecircumstances the additive from line 43a will flow into line 8 throughbypass line 10 controlled by valve 10a. When tank 45 and zone 46 arebypassed, valve 470 in line 47 is closed.

Weaker acid is discharged from filter zone 41 by lines 53 and 54. Theweaker acid withdrawn by line 53 may be introduced into line 33 to formthe recycle acid. This acid has a strength of about 10-15% H PO whilethe still weaker acid of about 5% H 1 0 withdrawn by line 54 may berecycled to filter zone 41 by line 55. Wash water may be introduced intofilter zone 41 by line 56 which connects to branch line 57 and by line58 controlled by valve 59 into line 55.

The phosphoric acid recovered by line 52 is discharged into a hold tank53 and from thence it is introduced by line 54 into centrifuge zonewhere additional solids are removed and the 54% acid further clarified.These solids are discharged from zone 55 by line 56. The clarified acidis then sent to storage by line 58 and may be withdrawn for use by line59. In storage 57 any solids not removed in zone 55 may settle out andfurther clarify the acid. From time to time, therefore, deposits ofsolids and sediment, if any, in tank 57 may be removed by means notshown.

It will be seen by reference to the drawing that a process has beenprovided for clarification of acid produced in a wet phosphoric acidprocess by. injecting a particular clarification additive into one ormore stages of the digestion zone and/or into acid introduced into theevaporation zone or into the centrifugation zone.

In order to illustrate the invention further, the following specificexamples are given:

EXAMPLES Five hundred parts of filtered 35% phosphoric acid containingabout 26 percent by wt. of solids were introduced into a vessel suitablefor heating under reduced pressure while mixing. The phosphoric acid washeated to an evaporation temperature of about 215 F. at atmosphericpressure, and 25 ppm. of clarification additive (sulfonated polystyrenehaving a molecular weight of about 6 million) added. A reduced pressureof 260 mm. mercury was applied while mixing the contents of the vesseland the acid was concentrated to 54% 11 1 0 The pressure was raised toatmospheric pressure and a second increment of 25 ppm. of the sameclarification additive added and mixed for about 30 seconds. The treatedacid was then centrifuged. It was found that the phosphoric acidrecovered after centrifuging contained only 0.35 percent by wt. ofsolids as compared with phosphoric acid of comparable strength which hadnot been treated in accordance with the present invention and whichcontained 1.3 percent solids.

In another operation, 53% H PO, containing 3.8 percent by wt. solidswith no clarification additive was centrifuged and found to contain 1.3percent by wt. solids, no additive being employed.

III

In still another operation, 53% H PO containing 2.74 percent by wt. ofsolids had 50 ppm. of clarification additive (sulfonated polystyrene) of12 million mol. wt. added. The acid was then centrifuged and theclarified acid was found to contain only 0.35 percent by wt. of solids.

In this example, 30% H PQ, in the digestion zone of a wet phosphoricacid process was found to contain 25 percent by wt. of solids. Fiftyppm. of clarification additive was added to the acid from the digestionzone and after filtration the 54 percent acid recovered therefrom afterevaporation was found to contain only 0.12 percent by wt. solids. Theclarification additive was sulfonated polystyrene of 12 million mol. wt.

It is apparent from the several examples where the sulfonatedpolystyrene was used, unobvious results were obtained in removing solidsand providing an acid substantially free of solids which transmittedlight freely. The clarified acid had a pale straw color. The inventionis, therefore, quite useful since solids-free acid is more desirablethan that containing suspended or dispersed solids.

The nature and objects of the present invention having been fullydescribed and illustrated and the best mode contemplated set forth whatwe wish to claim as new and useful and secure by Letters Patent is:

1. In a wet process for producing clarified phosphoric acid fromphosphate rock by reaction of said rock with sulfuric acid having a rockdigestion zone, an evaporation zone, and having at least a centrifugezone or a storage zone which comprises:

introducing into the acid in at least one of said zones an effectiveamount within the range from about 10 to about 200 p.p.m. based on saidacid of a clarification additive selected from the group consisting ofsulfonated polystyrene and its water soluble salts having a molecularweight within the range of about 1 million to about 40 million andsulfonated in the para-position of the benzene ring in an amount ofabout 1 to about 3 moles per mole of benzene ring; and

removing solids from said acid containing said additive.

5. A process in accordance with claim 1 in which the clarificationadditive is introduced into the acid introduced into the evaporationzone.

6. A process in accordance with claim 1 in which the clarificationadditive is introduced into the acid discharged from the evaporationzone into the centrifuge zone.

7. A process in accordance with claim 1 in which the clarificationadditive is sulfonated polystyrene of a molecular weight of about 3 toabout 12 million 8. A process in accordance with claim 1 in which theclarification additive is introduced into the acid in the storage zone.

9. A process in accordance with claim 1 in which the clarificationadditive is introduced into each of said zones.

10. A process in accordance with claim 9 in which the amount ofclarification additive is introduced in increments.

l l. A process in accordance with claim 10 in which the clarificationadditive is introduced into each of said zones.

12. A process in accordance with claim 1 in which:

a. the molecular weight of the clarification additive is within 5 therange of about 3 to 12 million;

b. the clarification additive is introduced in increments; and c. theclarification additive is sulfonated polystyrene. 13. In theclarification of phosphoric acid containing suspended solids, the stepsof clarifying said acid which com- 0 prises:

adding to said phosphoric acid an effective amount within the range fromabout 10 to about 200 p.p.m. based on said acid of a clarificationadditive selected from the I 5 group consisting of sulfonatedpolystyrene and its water soluble salts having a molecular weight withinthe range of about 1 million to about 40 million and sulfonated in theparaposition of the benzene ring in an amount of about 1 to about 3moles per mole of benzene ring; and removing solids from said phosphoricacid containing said additive.

2. A process in accordance with claim 1 in which the clarificationadditive has a molecular weight within the range of about 3 million toabout 12 million.
 3. A process in accordance with claim 1 in which theclarification additive has a molecular weight of about 6 million.
 4. Aprocess in accordance with claim 1 in which the clarification additiveis introduced into the digestion zone.
 5. A process in accordance withclaim 1 in which the clarification additive is introduced into the acidintroduced into the evaporation zone.
 6. A process in accordance withclaim 1 in which the clarification additive is introduced into the aciddischarged from the evaporation zone into the centrifuge zone.
 7. Aprocess in accordance with claim 1 in which the clarification additiveis sulfonated polystyrene of a molecular weight of about 3 to about 12million.
 8. A process in accordance with claim 1 in which theclarification additive is introduced into the acid in the storage zone.9. A process in accordance with claim 1 in which the clarificationadditive is introduced into each of said zones.
 10. A process inaccordance with claim 9 in which the amount of clarification additive isintroduced in increments.
 11. A process in accordance with claim 10 inwhich the clarification additive is introduced into each of said zones.12. A process in accordance with claim 1 in which: a. the molecularweight of the clarification additive is within the range of about 3 to12 million; b. the clarification additive is introduced in increments;and c. the clarification additive is sulfonated polystyrene.
 13. In theclarification of phosphoric acid containing suspended solids, the stepsof clarifying said acid which comprises: adding to said phosphoric acidan effective amount within the range from about 10 to about 200 p.p.m.based on said acid of a clarification additive selected from the groupconsisting of sulfonated polystyrene and its water soluble salts havinga molecular weight within the range of about 1 million to about 40million and sulfonated in the paraposition of the benzene ring in anamount of about 1 to about 3 moles per mole of benzene ring; andremoving solids from said phosphoric acid containing said additive.